Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in an internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element is mounted to the end of an engine's camshaft, including intake or exhaust valve camshafts in engines having dual camshafts.
In the prior art, cam phasers typically employ one of two different arrangements for achieving variable valve timing.
In a first arrangement, the sprocket element is provided with a first cylinder having helical splines on its inner surface, and the camshaft element is provided with a second cylinder having helical splines on its outer surface. The first and second cylinders nest together. When one cylinder is driven axially of the other, the helical splines cause relative rotation there between, thereby changing the phase relationship. Typically, an axially-acting ram is controllably displaced by pressurized engine oil pirated from the engine oil supply system.
In a second arrangement, the sprocket element is provided with a stator having a central opening and having a plurality of lobes extending radially inward into the central opening and spaced apart angularly of the stator body. The camshaft element is provided with a rotor having hub and a plurality of outwardly extending vanes. When the rotor is installed into the stator, the vanes are disposed between the lobes, thereby defining a plurality of rotor-advancing chambers on first sides of the vanes and a plurality of rotor retarding chambers on the opposite sides of the vanes. Again, pressurized oil is controllably admitted to either the advance chambers or the retard chambers to selectively alter the phase angle between the crankshaft and the camshaft, thereby varying the timing of the engine valves.
While effective and relatively inexpensive, both types of prior art cam phasers suffer from several drawbacks.
First, at low engine speeds engine oil pressure tends to be low, and sometimes unacceptably so; therefore, the response of a conventional cam phaser is sluggish at low engine speeds.
Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump and to supply to a phaser in a rapid-response fashion.
Third, using engine oil to drive a phaser is parasitic on the engine oil system and can lead to requirement for a larger oil pump.
And finally, for fast actuation, a larger engine oil pump may be necessary, resulting in an additional energy drain on the engine.
What is needed in the art is a camshaft phaser wherein the phaser is not actuated by pressurized oil and therefore phaser performance is not subject to variation in engine oil pressure, temperature, or viscosity.
It is a principal object of the present invention to vary engine valve timing by varying camshaft phase angle without reliance on pressurized oil.